Decreased incorporation of the photoaffinity probe 8N3-[gamma-32P]-GTP into a 45kD protein in lung tumors

Cyclin D1 as a proliferative marker regulating retinoblastoma phosphorylation in mouse lung epithelial cells

Elevations in cyclin D1 content increase the phosphorylation status of retinoblastoma (Rb) protein to encourage cell cycle transit. We sought to determine if cyclin D1 content could be used as an index of cell proliferation in mouse lung epithelia following growth manipulations in vitro and in vivo. Rb protein concentration was high in 82-132 and LM2, two fast-growing neoplastic mouse lung epithelial cell lines. The hyperphosphorylated form of Rb predominated in these two cell lines, while Rb in slower-growing cell lines was predominantly hypophosphorylated. Consistent with this, more cyclin D1 protein was expressed in the fast-growing cell lines than in slower-growing cells. We therefore tested whether cyclin D1 content varied with growth status. The amount of cyclin D1 decreased upon serum removal coincident with growth inhibition and then increased upon serum re-addition which stimulated resumption of proliferation. This correlation between cyclin D1 content and growth status also occurred in vivo. Cyclin D1 content increased when lungs underwent compensatory hyperplasia following damage caused by butylated hydroxytoluene administration to mice and in lung tumor extracts as compared with extracts prepared from uninvolved tissue or control lungs. We conclude that elevated cyclin D1 levels account, at least in part, for the hyperphosphorylation of Rb in neoplastic lung cells, and are associated with enhanced lung growth in vitro and in vivo.

Mouse lung epithelial cell lines--tools for the study of differentiation and the neoplastic phenotype

Several dozen lung epithelial cell lines have been established in culture over the past 20 years from normal lung explants and their spontaneous transformants, and from lung tumors that arose spontaneously or were induced with chemicals, viruses, or oncogenic transgenes. To provide information from which to choose appropriate lines for investigating problems in lung cell biology and pulmonary neoplasia, this review describes the origins of these lines and some of their characteristics. These include growth, morphology, tumorigenicity, ability to metastasize, xenobiotic metabolism, mutational status, signal transducing activities, cytogenetics, ability to form domes, and electric conductance. In addition to collecting this information in a single place for the first time, we describe previously unpublished apoptosis features of some of these lines. An increasing number of investigations are beginning to use these lines and this review contains references into 1997.

Differential responsiveness to agents which stimulate cAMP production in normal versus neoplastic mouse lung epithelial cells

We examined the responsiveness of normal and neoplastic lung cells to agents which stimulate cAMP production. While their basal cAMP levels were similar, spontaneous in vitro transformant E9 cells and tumor-derived PCC4 cells produced much less cAMP in response to 1 microM isoproterenol compared to non-tumorigenic C10 cells derived from normal mouse lung epithelium. Iodocyanopindolol binding studies indicated that both neoplastic lines contained fewer beta-adrenergic receptors than normal C10 cells. When receptors were bypassed via treatment with 10 pM cholera toxin, the pattern of cAMP-responsiveness was reversed; both neoplastic cell lines produced more cAMP than C10 cells. Direct stimulation of adenylate cyclase with 100 microM forskolin greatly increased cAMP concentrations in all three cell lines. These anomalies at both the receptor and G-protein levels in neoplastic lung epithelial cells may contribute to their deregulated growth.

Alterations in the cAMP signal transduction pathway in mouse lung tumorigenesis

Alterations in the cAMP signal transduction pathway are associated with mouse lung neoplasia, cAMP effects are mediated by activating cAMP-dependent protein kinase isozymes, PKA I and PKA II. E9, a tumorigenic cell line, exhibited decreased PKA I levels compared to C10 cells, a nontumorigenic cell line of similar epithelial origin. Western immunoblots of PKA subunit proteins demonstrated low concentrations of both the catalytic (C) and regulatory (RI) PKA I subunits. Although RII (regulatory subunit of PKA II) concentrations were similar in both cell lines, RII from E9 cells was more highly phosphorylated than in C10 cells. RII phosphorylation status regulates cAMP activation of PKA II. Northern-blot analysis of mRNA content indicated diminished expression of both C and RI mRNA in E9 relative to C10 cells. Several endogenous PKA substrate proteins present in C10 cells were minimally phosphorylated by PKA in E9 cells. Forskolin, which raises cellular cAMP content, increased phosphorylation of a protein doublet in intact C10 cells, but not in E9 cells. Decreased PKA I expression and alterations in RII phosphorylation in lung neoplasia may contribute to anomalous regulation by cAMP, thereby diminishing cAMP-mediated growth inhibitory effects.

Mechanisms of glucocorticoid involvement in mouse lung tumorigenesis

This report examines a possible mechanism of mouse lung tumor prophylaxis by glucocorticoids (GC). Adrenalectomy (Ax) increased, and corticosterone replacement decreased, lung tumor multiplicity when treatment was begun before administration of the carcinogen, urethan. Ax increased the 3H-thymidine labeling index of alveolar epithelial cells. Tumor multiplicity was also enhanced when urethan was administered during the period of compensatory hyperplasia that occurred in response to lung injury induced by methylcylopentadienyl manganese tricarbonyl. Thus, carcinogen-induced tumor development was amplified by stimulation of division of the target cell population. GC regulation of alveolar epithelial cell proliferation, and hence tumor susceptibility, may be mediated by the Ca++/phospholipid-dependent protein kinase (PKC).The tumor-resistant strain, C57BL/6J, has greater adrenal corticosterone content, higher epithelial cell PKC activity, and lower alveolar epithelial cell proliferation than the tumor-susceptible strain, A/J. In vitro, GC inhibit proliferation of a lung epithelial-derived cell line and increase PKC activity in that cell line. Thus, we hypothesize that GC protect against lung tumor development by increasing PKC content in the epithelial cells from which lung tumors arise; increased intracellular PKC results in decreased epithelial proliferation, and reduces the probability of induction of tumorigenesis by urethan.

Enhancement of GTP gamma S-binding activity by cAMP-dependent phosphorylation of a filamin-like 250 kDa membrane protein in human platelets

The treatment of human platelets with the dibutyryl cyclic AMP (dbcAMP) revealed the presence of a 250 kDa protein which enhanced its GTP-binding activity. This protein was purified from platelet membranes by successive chromatographies on DEAE-cellulose, Ultrogel AcA34, Mono Q, HCA-hydroxyapatite, and TSK-3000SW columns. The positive cross-reaction of the 250 kDa protein with the anti-filamin antibody indicated that this protein is filamin or very close to it. The GTP gamma S-binding activity of this protein, when phosphorylated with cyclic AMP-dependent protein kinase (A-kinase), showed an over tenfold increase, with the specific activity being 3.6 nmol/mg protein. Dephosphorylation of the phosphorylated protein with alkaline phosphatase reduced the GTP gamma S-binding activity to the control untreated level.

Decreased 8N3-[gamma-32P]GTP photolabeling of Gs alpha in tumorigenic lung epithelial cell lines: association with decreased hormone responsiveness and loss of contact-inhibited growth

The 45-kDa alpha subunit of the signal transducing Gs protein complex, which stimulates receptor-coupled adenylate cyclase, incorporated less of the photoaffinity probe, 8N3-[gamma-32P]GTP, in extracts from tumorigenic cell lines in comparison with nontumorigenic cell lines derived from mouse lung epithelium. Immunoblotting experiments using anti-Gs alpha antibodies demonstrated that tumor cells do not have a decreased amount of Gs alpha and photolabeling of tumor cell Gs alpha increased when the rate of nucleotide exchange was promoted. Therefore, tumor cell Gs alpha function may be altered. Consistent with this hypothesis is the observation that the tumor cells exhibited decreased responsiveness to the beta-adrenergic agonist, isoproterenol. Gs alpha photolabeling in growing nontumorigenic cells was reduced to a level resembling that observed in tumor cells, but photolabeling increased when cells became contact-inhibited. This increase in 8N3-[gamma-32P]GTP incorporation into Gs alpha by normal cells at confluence was not seen in the tumorigenic cells. Since Gs alpha photolabeling was inversely proportional to the percentage of [3H]thymidine-labeled nuclei at confluence, we suggest that the altered Gs alpha in tumor cells is involved in the loss of cell growth regulation.

Inhibition by cyclic AMP of guanine nucleotide-induced activation of phosphoinositide-specific phospholipase C in human platelets

Phosphoinositide-specific phospholipase C (PLC) activity of human platelet membranes was activated by the nonhydrolyzable guanine nucleotide GTP gamma S. This activation did not occur in either membranes prepared from dibutyryl cyclic AMP-pretreated platelets (A-membranes) or those prepared from untreated cells and subsequently incubated with cyclic AMP (cAMP) (B-membranes). This cAMP-mediated inhibition was abolished in the presence of inhibitors of cAMP-dependent protein kinase (A-kinase), suggesting that the inhibition was due to phosphorylation of (a) protein component(s). No significant differences were observed in the basal PLC activity and the extent of pertussis toxin-catalyzed ADP-ribosylation among control membranes and the two types of phosphorylated membranes (A- and B-membranes). GTP-binding activities of Gs, Gi and GTP-binding proteins of lower molecular masses were not altered by the phosphorylation of the membranes. These findings suggest that a GTP-binding protein is involved in the GTP gamma S-mediated activation of PLC and that cAMP (plus A-kinase) inhibits this activation by phosphorylating a membrane protein (probably a 240-kDa protein), rather than the GTP-binding protein or PLC itself. It is likely that this phosphorylation uncouples the GTP-binding protein from PLC.

Platelet cytosolic 44-kDa protein is a substrate of cholera toxin-induced ADP-ribosylation and is not recognized by antisera against the alpha subunit of the stimulatory guanine nucleotide-binding regulatory protein

ADP-ribosylation induced by cholera toxin and pertussis toxin was studied in particulate and cytosolic fractions of human platelets. Platelets were disrupted by a cycle of freezing and thawing in the presence of a hyposmotic buffer containing protease inhibitors. In both fractions, the A subunit of cholera toxin ADP-ribosylates two proteins with molecular masses of 42 and 44 kDa, whereas pertussis toxin ADP-ribosylates a 41-kDa polypeptide. Two antisera against the alpha subunit of the stimulatory guanine nucleotide-binding regulatory protein recognize only the 42-kDa polypeptide. Cholera toxin-induced ADP-ribosylation of the 42- and 44-kDa proteins is reduced by pretreatment of platelets with iloprost, a prostacyclin analog. The 44-kDa protein, which is substrate of cholera toxin, could be extracted completely from the membrane and recovered in the cytosolic fraction when the cells were disrupted by Dounce homogenization and the pellet was extensively washed. A 44-kDa protein can also be labeled with 8-azidoguanosine 5'-[alpha-32P]triphosphate in the cytosol and membranes. These findings indicate that cholera and pertussis toxins produced covalent modifications of proteins present in particulate and cytosolic platelet fractions. Moreover, the 44-kDa protein might be an alpha subunit of a guanine nucleotide-binding regulatory protein that is not recognized by available antisera.